Biomarker characteristics of Jurassic source rocks in Angdarco area in Qiangtang Basin and their significance

The characteristics of biomarkers for three sets of main source rocks in Angdarco area of the Qiangtang Basin were studied, and the significance for sedimentary environment and thermal maturity was discussed, based on the analyses of saturated hydrocarbon chromatogram and mass spectrometry. Thermally mature source rocks from the Lower Jurassic Quse Formation have relatively higher ratios of (C₂₁+C₂₂)/(C₂₈+C₂₉), Ts/Tm, C₃₁ homohopane S/R, and rearranged sterane/regular sterane. The main sources were sapropelic and humic-sapropelic marine organic matter. Source rocks from the Middle Jurassic Buqu Formation were deposited in a reducing environment, and were distinguished from the other two source rocks by relatively lower ratios of Pr/Ph and C₁₉/C₂₃ tricyclic terpane, which indicated sapropelic and humic-sapropelic organic matter inputs, at low mature to mature stage. Source rocks from the Middle Jurassic Xiali Formation were deposited in a marine-terrestrial transitional environment with a lower salinity, presenting humic and sapropelic-humic organic matter inputs, and low thermal maturity.